[GHC] #8161: Associated type parameters that are more specific than the instance header

[GHC]

#8161: Associated type parameters that are more specific than the instance header
-------------------------------------+-------------------------------------
       Reporter:  adamgundry         |             Owner:
           Type:  feature request    |            Status:  new
       Priority:  low                |         Milestone:
      Component:  Compiler (Type     |           Version:  7.7
  checker)                           |  Operating System:  Unknown/Multiple
       Keywords:                     |   Type of failure:  GHC rejects
   Architecture:  Unknown/Multiple   |  valid program
     Difficulty:  Unknown            |         Test Case:
     Blocked By:                     |          Blocking:
Related Tickets:                     |
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 It would be nice if type parameters of associated types could be more
 specific than those in the instance header. This is currently rejected
 with the message "Type indexes must match class instance head", but could
 be accepted:

 {{{
 {-# LANGUAGE TypeFamilies #-}

 class C a where
   type T a

 instance C [a] where
   type T [Bool] = Int
   type T [Int]  = Int
 }}}

 More typically, this is useful where we want to use an equality constraint
 to make type inference easier, but need to match on the actual type in an
 associated type:

 {{{
 {-# LANGUAGE TypeFamilies, FlexibleInstances #-}

 class C a where
   type T a

 instance a ~ [b] => C a where
   type T [b] = Int
 }}}

 This showed up in the implementation of
 [wiki:Records/OverloadedRecordFields/Plan OverloadedRecordFields]. Of
 course, one can always work around it using a normal (non-associated) type
 family.

 Note that we already allow type families to specialise variables that do
 not occur in the instance header:

 {{{
 {-# LANGUAGE TypeFamilies #-}

 class C a where
   type T a b

 instance C [a] where
   type T [a] [Bool] = Bool
   type T [a] [Int]  = Int
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8161>
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